This invention relates to pressure relief devices and is especially but not exclusively concerned with such devices for use with closed vessels such as gas cylinders and fire extinguishers.
By a "pressure relief device" is meant a device which will permit the escape of fluid (liquid, vapour or gas) from a system so as to relieve the pressure therein when the difference between the fluid pressure in the system acting on one side of the device and the atmospheric or other pressure acting on the other side reaches a predetermined value or a value within a predetermined range of values.
Known pressure relief devices include so-called "normal" and "reverse" bursting discs. These are generally made of metal such as stainless steel or an aluminium alloy and consist of a domed central portion which is integral with a surrounding flat annular flange.
In use, this flange is clamped tightly between first and second metal collars normally by means of threaded bolts which pass through the first collar and screw into tapped holes in the second, the heads of the bolts being recessed below the surface of the first collar and the ends of the bolts, when screwed home, not projecting beyond the surface of the second. In any field of use, the interaction between the confronting surfaces of bursting disc and the collars is a contributory factor in bursting disc performance.
The resulting unitary combination of bursting disc and collars is, in turn, clamped in a fluid-tight manner between a pair of pipe flanges and, with normal bursting discs, the pressure is arranged to act on the concave surface of the disc so that in the event of the pressure exceeding a predetermined value, or a value within a predetermined range of values, the disc bursts outwardly so as to relieve the pressure.
With a reverse bursting disc, or "buckling" disc as it is sometimes called, the pressure acts on the convex surface. This type of bursting disc relieves the pressure by collapsing, generally on to a cruciform array of knives, which ensure that the dome breaks up into "petals" and thereby effects a positive relief of the pressure.
Bursting disc assemblies of the type just described are widely used in pipe-lines in chemical installations but are generally too large and expensive for use on gas cylinders, fire extinguishers and the like, although, in these applications, the positive relief of pressure which a bursting disc assembly can provide is highly desirable.
Bursting disc arrangements for use with pressurisable containers such as gas cylinders and the containers used in fire extinguishers are accordingly generally cheaper and simpler than the assemblies just described and frequently take the form of a small flat disc, usually of metal, which is secured in a seat in the base of a generally cylindrical housing welded or otherwise secured to the wall of the container. The base of the housing is provided with one or more channels which pass through the said base and communicate with the interior of the container, the disc being secured in its seat in the housing by means of a tubular holding member which is threaded on the outside and is screwed into the housing so that its inner end bears on at least the peripheral regions of the disc. In this way the outer annular portion of the disc is pressed on to the seating so as to form a fluid-tight seal therewith and to prevent the escape of any fluid introduced into the container at a pressure less than the bursting pressure of the disc.
The pressure at which the disc will burst or rupture depends inter alia upon the temperature, the dimensions of the disc, the material from which it is made, the physical state of this material and the dimensions of the bore or lumen of the tubular member which holds the disc in place and which thereby governs the deformable area of the disc as well as the clamping load applied to the disc by the tubular member.
As the pressure approaches the critical bursting pressure, the central portion (that is, the said "deformable area") of the disc tends to be deformed into a dome outwardly, that is away from the container, and, to minimise the possibility of unwanted mechanical damage to the disc during this process, the inner edge of that end of the tubular holding member which bears on the disc is rounded. Alternatively, the corresponding edge of a collar or washer, which may be interposed between the said end of the tubular member and the disc, is similarly rounded.
In order to ensure that the disc will burst at a predetermined pressure or at a pressure within a predetermined range of pressures, such as, for example, a pressure within 10% of 225 bar (about 3300 psi), the composition and physical condition of the material of the disc are very carefully controlled as are the dimensions of the disc and of the bore of the cylindrical member which holds the disc in place. In spite of these precautions, we have found that in assemblies of the type just described, the disc performance will often vary in a random and unpredictable manner which is quite unacceptable. Disc leakage and incorrect bursting pressure are the most commonly occurring faults.
A further disadvantage of assemblies of the type just described is that at filling stations for gas cylinders, for example, it is common practice for the operators to replace the bursting discs in the pressure relief devices of the cylinders before they are filled and it is not unknown on these occasions for two discs rather than one to be placed accidentally in the seats of one or more of these devices. These are dangerous mishaps which can have disastrous consequences.
Investigations have shown us that the unpredictable bursting pressures of the discs previously referred to are due, at least in part, to annular torque or shearing stresses set up in the discs when the tubular holding members are screwed down on them as a result of the tendency of the ends of the holding members to turn the discs in their seats as they are screwed down and the tendency of the seats to resist this turning movement.
For convenience, arrangements in which bursting discs may be clamped against a seat, for example, in a bursting disc assembly without being subject to the shearing stresses just referred to, will be described as "torque-free" or torque-insensitive throughout the remainder of this specification.